In known conventional time domain signal analysis techniques, various engine transducers (e.g. exhaust pressure, intake manifold vacuum, oil pressure, crankcase pressure and engine speed) produce periodic electronic signals which define certain engine operating conditions (e.g. intake/exhaust valve opening and closing, cylinder combustion quality, exhaust cylinder blowdown, etc.). The fundamental frequency of these signals is termed the engine's firing frequency and is defined by the following equation: EQU FIRING FREQUENCY (Hz)=[ENGINE SPEED (RPM).times.(N/2)
Where N=number of cylinders
As indicated by the above equation, the fundamental frequency of the signals is a function of engine speed and therefore the electronic signals change frequency as engine speed is increased or decreased. Each engine firing includes two crankshaft revolutions (through 720.degree.) and, for a four cylinder engine, four cylinder firings or, for an eight cylinder engine, eight cylinder firings. In time domain signal analysis, time records as a function of engine crank angle are stored in a computer for the various transducers for an engine containing no defects and operated at a constant speed (e.g. 1000 RPM) in order to establish baseline waveforms. A second time record is taken for a defective engine for comparison to the baseline "normal" waveform. Comparing the two time records, a discriminant is extracted from the data to represent the particular defect as shown in FIG. 1. By determining defects by this method (data as a function of crankangle), defects can also be isolated to a particular cylinder. In this manner, a discriminant can be defined for many engine defects by implanting known defects in normal engines and repeating the above tests. This method can require a considerable amount of data storage and manipulation.
In addition to time domain analysis, frequency domain analysis is also known. For example, U.S. Pat. No. 4,083,234 to Aono et al teaches a method and apparatus for detecting a misfire in an internal combustion engine. A cylinder misfiring is detected by initially sensing the exhaust noises and producing a continuous signal wave having an exhaust noise frequency which is variable with the detected exhaust noises. The waveform has a frequency spectrum having peak values at various frequencies. One such peak exists at the fundamental frequency of the wave and represents a normal operating condition. Other peak values occur at certain fractional harmonics of the fundamental frequency component and represent a misfire of an engine cylinder. The system also incorporates an engine speed signal which, in combination with a frequency spectrum analyzer, aids in the evaluation of the misfired cylinder. In operation, the speed signal is used to control the band pass filtering of the exhaust noise waveform. This patent shows a misfire detection technique using band pass filtering and subsequent frequency spectrum analysis of a noise signal representing exhaust noise. This patent does not teach detection of engine faults other than misfire. Further, there is no teaching of using a plurality of different sensors to identify engine operating conditions. These are some of the problems this invention overcomes.